Articles for manipulating impinging liquids and methods of manufacturing same

ABSTRACT

This invention relates generally to an article that includes a non-wetting surface having a dynamic contact angle of at least about 90°. The surface is patterned with macro-scale features configured to induce controlled asymmetry in a liquid film produced by impingement of a droplet onto the surface, thereby reducing time of contact between the droplet and the surface.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of, and incorporatesherein by reference in its entirety U.S. Provisional Patent ApplicationNo. 61/514,794, which was filed on Aug. 3, 2011.

FIELD OF THE INVENTION

This invention relates generally to surfaces that manipulate impingingliquids. More particularly, in certain embodiments, the inventionrelates to macro-scale features on a surface that reduce the contacttime between an impinging liquid and the surface.

BACKGROUND OF THE INVENTION

Superhydrophobicity, a property of a surface when it resists contactwith water, has been a topic of intense research during the last decadedue to its potential in a wide variety of applications, such asself-cleaning, liquid-solid drag reduction, and water repellency. Waterrepellency of superhydrophobic surfaces is often studied by dropletimpingement experiments in which millimetric drops of water are impactedonto these surfaces and photographed. With appropriate surface design,droplets can be made to bounce off completely. However, the time takento bounce off—hereafter referred to as the contact time—is criticallyimportant as mass, momentum, and/or energetic interactions take placebetween the droplet and the surface during the time of contact. Forexample, the energy required to deice an airplane wing can be reduced ifa water drop rebounds off the wing before it freezes.

Recent literature suggests there is a theoretical minimum contact time,t_(c). See M. Reyssat, D. Richard, C. Clanet, and D. Quere, FaradayDiscuss., 2010, 146, pp. 19-33; and D. Quere, Nature Letters, 2002, 417,pp. 811. Specifically, models that estimate the effects of contact linepinning on contact time have found that the contact time scales as

$\begin{matrix}{t_{c} \approx {2.2( \frac{\rho \; R^{3}}{\gamma} )^{1/2}( {1 + \frac{\varphi}{4}} )}} & (1)\end{matrix}$

where t_(c) is the contact time of a drop, of radius R, density ρ, andsurface tension γ, bouncing on a superhydrophobic surface with pinningfraction φ. Even if one were able to completely eliminate this surfacepinning such that φ=0, there would still be a minimum contact timelimited by the drop hydrodynamics.

New articles, devices, and methods are needed to decrease the contacttime between a droplet and a surface for improved liquid repellency.Contact times less than the theoretical minimum have heretofore beenbelieved to be impossible.

SUMMARY OF THE INVENTION

The articles, devices, and methods presented herein incorporate uniquesurface designs that can manipulate the morphology of an impingingdroplet and lead to a significant reduction (e.g., more than 50% belowthe theoretical minimum prediction of Equation 1) in the time of contactbetween a droplet and its target surface. These designs are capable ofimproving the performance of a wide variety of products that arenegatively affected by droplet impingement. Examples of such productsinclude rainproof consumer products, steam turbine blades, wind turbineblades, aircraft wings, engine blades, gas turbine blades, atomizers,and condensers.

The articles, devices, and methods described herein offer severaladvantages over previous approaches in the field of water repellencyusing superhydrophobic surfaces. For example, the articles, devices, andmethods lead to a major reduction (e.g., over 50%) in the contact timecompared to the existing best reported contact time in the literature(i.e., the minimum contact time predicted by Equation 1, above). Thissurprising reduction in contact time is desirable not only to controldiffusion of mass, momentum, or energy (depending upon the application),but also to prevent droplets from getting stuck on a surface due toimpact from neighboring impinging droplets. In addition, the approachdescribed herein is more practical and scalable as it relies onintroducing macro-scale features that are easy to machine or fabricatewith current tools. By contrast, previous approaches focus on the use ofmicron to sub-micron features that are difficult to fabricate and, atbest, provide contact times that approach but do not fall below theminimum predicted by Equation 1. Contact times achieved using thearticles, devices, and methods described herein are lower than thoseattainable with the lotus leaf (the best known superhydrophobicsurface), which is limited by Equation 1.

The articles, devices, and methods described herein may be used in awide variety of industries and applications where droplet repellency isdesirable. For example, textile companies that manufacture rainprooffabrics, such as rainwear, umbrellas, automobile covers, etc., couldsignificantly improve fabric waterproof performance. Likewise, energycompanies that manufacture steam turbines could reduce moisture-inducedefficiency losses caused by water droplets entrained in steam, whichimpinge on turbine blades and form films, thereby reducing power output.Condensers in power and desalination plants may utilize the devices andmethods described herein to promote dropwise shedding condensation heattransfer. Further, in aircraft and wind turbine applications, a reducedcontact time of supercooled water droplets impinging upon aircraftsurfaces is desirable to prevent the droplets from freezing and therebydegrading aerodynamical performance. In atomizer applications, theability of surfaces to break up droplets can be used to create newatomizers for applications in engines, agriculture, and pharmaceuticalindustries. In gas turbine compressors, the devices and methodsdescribed herein may be used to prevent oil-film formation and reducefouling.

In one aspect, the invention relates to an article including anon-wetting surface having a dynamic contact angle of at least about90°, said surface patterned with macro-scale features configured toinduce controlled asymmetry in a liquid film produced by impingement ofa droplet onto the surface, thereby reducing time of contact between thedroplet and the surface. In certain embodiments, the non-wetting surfaceis superhydrophobic, superoleophobic, and/or supermetallophobic. In oneembodiment, the surface includes a non-wetting material. The surface maybe heated above its Leidenfrost temperature.

In certain embodiments, the surface includes non-wetting features, suchas nanoscale pores. In certain embodiments, the macro-scale featuresinclude ridges having height A_(r) and spacing λ_(r), with A_(r)/hgreater than about 0.01 and λ_(r)/A_(r) greater than or equal to about1, wherein h is lamella thickness upon droplet impingement onto thesurface. In certain embodiments, A_(r)/h is from about 0.01 to about 100and λ_(r)/A_(r) is greater than or equal to about 1. In one embodiment,A_(r)/h is from about 0.1 to about 10 and λ_(r)/A_(r) is greater than orequal to about 1.

In certain embodiments, the article is a wind turbine blade, themacro-scale features include ridges having height A_(r) and spacingλ_(r), and wherein 0.0001 mm<A_(r) and λ_(r)≧0.0001 mm. In certainembodiments, the article is a rainproof product, 0.0001 mm<A_(r) andλ_(r)>0.0001 mm. In some embodiments, the article is a steam turbineblade, 0.00001 mm<A_(r) and λ_(r)>0.0001 mm. In one embodiment, thearticle is an exterior aircraft part, 0.00001 mm<A_(r) and λ_(r)>0.0001mm. The article may be a gas turbine blade with 0.00001 mm<A_(r) andλ_(r)>0.0001 mm.

In certain embodiments, the macro-scale features include protrusionshaving height A_(p) and whose centers are separated by a distance λ_(p),with A_(p)/h>0.01 and λ_(p)/A_(p)≧2, wherein h is lamella thickness upondroplet impingement onto the surface. In certain embodiments,100>A_(p)/h>0.01 and λ_(p)/A_(p)≧2. In one embodiment, 10>A_(p)/h>0.1and λ_(p)/A_(p)≧2. The macro-scale features may be hemisphericalprotrusions.

In certain embodiments, the article is a wind turbine blade, themacro-scale features include protrusions having height A_(p) and whosecenters are separated by a distance λ_(p), and wherein 0.0001 mm<A_(p)and λ_(p)≧0.0002 mm. In certain embodiments, the article is a rainproofproduct, 0.0001 mm<A_(p) and λ_(p)≧0.0002 mm. In various embodiments,the article is a steam turbine blade, 0.00001 mm<A_(p) and λ_(p)≧0.00002mm. In certain embodiments, the article is an exterior aircraft part,0.00001 mm<A_(p) and λ_(p)≧0.00002 mm. The article may be a gas turbineblade with 0.00001 mm<A_(p) and λ_(p)≧0.00002 mm.

In certain embodiments, the macro-scale features include a sinusoidalprofile having amplitude A_(c) and period λ_(c), with A_(c)/h>0.01 andλ_(c)/A_(c)≧2, wherein h is lamella thickness upon droplet impingementonto the surface. In certain embodiments, 100>A_(c)/h>0.01 and500≧λ_(c)/A_(c)>2. In various embodiments, 100>A_(c)/h>0.1 and500≧λ_(c)/A_(c)≧2. As used herein, “sinusoidal” encompasses any curvedshape with an amplitude and period.

In certain embodiments, the article is a rainproof product, themacro-scale features include a sinusoidal profile having amplitude A_(c)and period λ_(c), and wherein 0.0001 mm<A_(c) and λ_(c)≧0.0002 mm. Inone embodiment, the article is a wind turbine blade, 0.0001 mm<A_(c) andλ_(c)≧0.0002 mm. The article may be a steam turbine blade with 0.00001mm<A_(c) and λ_(c)≧0.00002 mm. The article may be an exterior aircraftpart with 0.00001 mm<A_(c) and λ_(c)≧0.00002 mm. In certain embodiments,the article is a gas turbine blade, 0.00001 mm<A_(c) and λ_(c)≧0.00002mm.

In certain embodiments, the surface includes an alkane. In oneembodiment, the surface includes a fluoropolymer. In certainembodiments, the surface includes at least one member selected from thegroup consisting of teflon, trichloro(1H,1H,2H,2H-perfluorooctyl)silane(TCS), octadecyltrichlorosilane (OTS),heptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane, fluoroPOSS, aceramic material, a polymeric material, a fluorinated material, anintermetallic compound, and a composite material. In certainembodiments, the surface includes a polymeric material, the polymericmaterial including at least one of polytetrafluoroethylene,fluoroacrylate, fluoroeurathane, fluorosilicone, fluorosilane, modifiedcarbonate, chlorosilanes, and silicone. In certain embodiments, thesurface includes a ceramic material, the ceramic material including atleast one of titanium carbide, titanium nitride, chromium nitride, boronnitride, chromium carbide, molybdenum carbide, titanium carbonitride,electroless nickel, zirconium nitride, fluorinated silicon dioxide,titanium dioxide, tantalum oxide, tantalum nitride, diamond-like carbon,and fluorinated diamond-like carbon. In certain embodiments, the surfaceincludes an intermetallic compound, the intermetallic compound includingat least one of nickel aluminide and titanium aluminide. In certainembodiments, the article is a condenser. The article may be a dripshield for storage of radioactive material. In certain embodiments, thearticle is a self-cleaning solar panel.

In another aspect, the invention relates to an atomizer including anon-wetting surface having a dynamic contact angle of at least about90°, said surface patterned with macro-scale features configured toinduce controlled asymmetry in a liquid film produced by impingement ofa droplet onto the surface, thereby promoting breakup of the droplet onthe surface. The description of elements of the embodiments above can beapplied to this aspect of the invention as well. In certain embodiments,the non-wetting surface is supermetallophobic. In certain embodiments,the droplet includes a molten metal.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be better understood withreference to the drawings described below, and the claims. The drawingsare not necessarily to scale, emphasis instead generally being placedupon illustrating the principles of the invention. In the drawings, likenumerals are used to indicate like parts throughout the various views.

While the invention is particularly shown and described herein withreference to specific examples and specific embodiments, it should beunderstood by those skilled in the art that various changes in form anddetail may be made therein without departing from the spirit and scopeof the invention.

FIG. 1 a is a schematic side view of a droplet resting on a surfaceduring a static contact angle measurement, according to an illustrativeembodiment of the invention.

FIGS. 1 b and 1 c are schematic side views of a liquid spreading andreceding, respectively, on a surface, according to an illustrativeembodiment of the invention.

FIG. 1 d is a schematic side view of a droplet resting on an angledsurface, according to an illustrative embodiment of the invention.

FIGS. 1 e and 1 f depict typical side and top views, respectively, of awater droplet (2.7 mm in diameter) impinging a superhydrophobic surface,according to an illustrative embodiment of the invention.

FIG. 2 a is a schematic top view of a droplet undergoing symmetricalrecoil, similar to FIG. 1 b, after impingement, according to anillustrative embodiment of the invention.

FIG. 2 b is a schematic top view of a droplet undergoing asymmetricrecoil due to nucleation of holes, according to an illustrativeembodiment of the invention.

FIG. 2 c is a schematic top view of a droplet undergoing asymmetricalrecoil due to development of cracks, according to an illustrativeembodiment of the invention.

FIG. 2 d is a schematic side view of a droplet that has spread onto acurved surface to form a lamella, according to an illustrativeembodiment of the invention.

FIG. 3 is a schematic side view and a detailed view of a surface fortriggering cracks in a receding liquid film, according to anillustrative embodiment of the invention.

FIG. 4 includes schematic top and cross-sectional views of a dropletrecoiling on a flat surface, according to an illustrative embodiment ofthe invention.

FIG. 5 includes schematic top and cross-sectional views of a dropletrecoiling on a ridge, according to an illustrative embodiment of theinvention.

FIGS. 6 a-6 c include top, cross-sectional, and high-magnificationscanning electron microscope (SEM) images of a macro-scale ridge (height˜150 μm, width 200 μm) fabricated on a silicon wafer usinglaser-rastering, according to an illustrative embodiment of theinvention.

FIG. 6 d includes high-speed photography images of droplet impingementon the ridge of FIGS. 6 a-6 c, according to an illustrative embodimentof the invention.

FIG. 7 a is an SEM image of a macro-scale ridge (height ˜100 μm, width˜200 μm) milled on an anodized aluminum oxide (AAO) surface, accordingto an illustrative embodiment of the invention.

FIG. 7 b is a high-magnification SEM image of the AAO surface of FIG. 7a, showing nanoscale pores, according to an illustrative embodiment ofthe invention.

FIG. 7 c includes high-speed photography images of droplet impingementon the ridge of FIG. 7 a, according to an illustrative embodiment of theinvention.

FIG. 8 is a schematic perspective view of macro-scale protrusions on asurface, according to an illustrative embodiment of the invention.

FIG. 9 a is an SEM image of macro-scale protrusions (˜50-100 μm)fabricated on anodized titanium oxide (ATO) surface, according to anillustrative embodiment of the invention.

FIG. 9 b is a high-magnification SEM image of the ATO surface of FIG. 9a showing nanoscale features, according to an illustrative embodiment ofthe invention.

FIG. 9 c includes high-speed photography images of droplet impingementon the surface of FIG. 9 a, according to an illustrative embodiment ofthe invention.

FIG. 10 includes a schematic cross-sectional view and a detailedschematic cross-sectional view of a surface having a macro-scalesinusoidal profile to trigger curvature in a receding liquid film,according to an illustrative embodiment of the invention.

FIG. 11 a includes a photograph showing a macro-scale sinusoidal surfacefabricated on silicon and an image showing high magnification SEMsub-micron features, according to an illustrative embodiment of theinvention.

FIG. 11 b includes high-speed photography images of droplet impingementon the surface of FIG. 11 a, according to an illustrative embodiment ofthe invention.

FIG. 12 a is a schematic view of droplet impingement on a solid surfaceat the instant of impact, according to an illustrative embodiment of theinvention.

FIG. 12 b is a schematic view of droplet impingement on a solid surfaceduring spreading, according to an illustrative embodiment of theinvention.

FIG. 12 c is a schematic view of droplet impingement on a solid surfaceat the instant when spreading comes to a rest, according to anillustrative embodiment of the invention.

DETAILED DESCRIPTION

It is contemplated that compositions, mixtures, systems, devices,methods, and processes of the claimed invention encompass variations andadaptations developed using information from the embodiments describedherein. Adaptation and/or modification of the compositions, mixtures,systems, devices, methods, and processes described herein may beperformed by those of ordinary skill in the relevant art.

Throughout the description, where devices and systems are described ashaving, including, or comprising specific components, or where processesand methods are described as having, including, or comprising specificsteps, it is contemplated that, additionally, there are systems of thepresent invention that consist essentially of, or consist of, therecited components, and that there are processes and methods accordingto the present invention that consist essentially of, or consist of, therecited processing steps.

Similarly, where devices, mixtures, and compositions are described ashaving, including, or comprising specific compounds and/or materials, itis contemplated that, additionally, there are mixtures and compositionsof the present invention that consist essentially of, or consist of, therecited compounds and/or materials.

It should be understood that the order of steps or order for performingcertain actions is immaterial so long as the invention remains operable.Moreover, two or more steps or actions may be conducted simultaneously.

The mention herein of any publication, for example, in the Backgroundsection, is not an admission that the publication serves as prior artwith respect to any of the claims presented herein. The Backgroundsection is presented for purposes of clarity and is not meant as adescription of prior art with respect to any claim.

Referring to FIG. 1 a, in certain embodiments, a static contact angle θbetween a liquid and solid is defined as the angle formed by a liquiddrop 12 on a solid surface 14 as measured between a tangent at thecontact line, where the three phases—solid, liquid, and vapor—meet, andthe horizontal. The term “contact angle” usually implies the staticcontact angle θ since the liquid is merely resting on the solid withoutany movement.

As used herein, dynamic contact angle, θ_(d), is a contact angle made bya moving liquid 16 on a solid surface 18. In the context of dropletimpingement, θ_(d) may exist during either advancing or recedingmovement, as shown in FIGS. 1 b and 1 c, respectively.

As used herein, a surface is “non-wetting” if it has a dynamic contactangle with a liquid of at least 90 degrees. Examples of non-wettingsurfaces include, for example, superhydrophobic surfaces andsuperoleophobic surfaces.

As used herein, contact angle hysteresis (CAH) is

CAH=θ_(a)−θ_(r)  (2)

where θ_(a) and θ_(r) are advancing and receding contact angles,respectively, formed by a liquid 20 on a solid surface 22. Referring toFIG. 1 d, the advancing contact angle θ_(a) is the contact angle formedat the instant when a contact line is about to advance, whereas thereceding contact angle θ_(r) is the contact angle formed when a contactline is about to recede.

As used herein, “non-wetting features” are physical textures (e.g.,random, including fractal, or patterned surface roughness) on a surfacethat, together with the surface chemistry, make the surface non-wetting.In certain embodiments, non-wetting features result from chemical,electrical, and/or mechanical treatment of a surface. In certainembodiments, an intrinsically hydrophobic surface may becomesuperhydrophobic when non-wetting features are introduced to theintrinsically hydrophobic surface. Similarly, an intrinsicallyoleophobic surface may become superoleophobic when non-wetting featuresare introduced to the intrinsically oleophobic surface. Likewise, anintrinsically metallophobic surface may become supermetallophobic whennon-wetting features are introduced to the intrinsically metallophobicsurface.

In certain embodiments, non-wetting features are micro-scale ornano-scale features. For example, the non-wetting features may have alength scale L_(n) (e.g., an average pore diameter, or an averageprotrusion height) that is less than about 100 microns, less than about10 microns, less than about 1 micron, less than about 0.1 microns, orless than about 0.01 microns. Compared to a length scale L_(m)associated with macro-scale features, described herein, the lengthscales for the non-wetting features are typically at least an order ofmagnitude smaller. For example, when a surface includes a macro-scalefeature that has a length scale L_(m) of 1 micron, the non-wettingfeatures on the surface have a length scale L_(n) that is less than 0.1microns. In certain embodiments a ratio of the length scale for themacro-scale features to the length scale for the non-wetting features(i.e., L_(m)/L_(n)) is greater than about 10, greater than about 100,greater than about 1000, or greater than about 10,000.

As used herein, a “superhydrophobic” surface is a surface having astatic contact angle with water of at least 120 degrees and a CAH ofless than 30 degrees. In certain embodiments, an intrinsicallyhydrophobic material (i.e., a material having an intrinsic contact anglewith water of at least 90 degrees) exhibits superhydrophobic propertieswhen it includes non-wetting features. For superhydrophobicity,typically nano-scale non-wetting features are preferred. Examples ofintrinsically hydrophobic materials that exhibit superhydrophobicproperties when given non-wetting features include: hydrocarbons, suchas alkanes, and fluoropolymers, such as teflon,trichloro(1H,1H,2H,2H-perfluorooctyl)silane (TCS),octadecyltrichlorosilane (OTS),heptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane, and fluoroPOSS.

As used herein, a “superoleophobic” surface is a surface having a staticcontact angle with oil of at least 120 degrees and a CAH with oil ofless than 30 degrees. The oil may be, for example, a variety of liquidmaterials with a surface tension much lower than the surface tension ofwater. Examples of such oils include alkanes (e.g., decane, hexadecane,octane), silicone oils, and fluorocarbons. In certain embodiments, anintrinsically oleophobic material (i.e., a material having an intrinsiccontact angle with oil of at least 90 degrees) exhibits superoleophobicproperties when it includes non-wetting features. The non-wettingfeatures may be random or patterned. Examples of intrinsicallyoleophobic materials that exhibit superoleophobic properties when givennon-wetting features include: teflon,trichloro(1H,1H,2H,2H-perfluorooctyl)silane (TCS),octadecyltrichlorosilane (OTS),heptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane, fluoroPOSS, andother fluoropolymers.

As used herein, a “supermetallophobic” surface is a surface having astatic contact angle with a liquid metal of at least 120 degrees and aCAH with liquid metal of less than 30 degrees. In certain embodiments,an intrinsically metallophobic material (i.e., a material having anintrinsic contact angle with liquid metal of at least 90 degrees)exhibits supermetallophobic properties when it includes non-wettingfeatures. The non-wetting features may be random or patterned. Examplesof intrinsically metallophobic materials that exhibit supermetallophobicproperties when given non-wetting features include: teflon,trichloro(1H,1H,2H,2H-perfluorooctyl)silane (TCS),octadecyltrichlorosilane (OTS),heptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane, fluoroPOSS, andother fluoropolymers. Examples of metallophobic materials include moltentin on stainless steel, silica, and molten copper on niobium.

In certain embodiments, intrinsically hydrophobic materials and/orintrinsically oleophobic materials include ceramics, polymericmaterials, fluorinated materials, intermetallic compounds, and compositematerials. Polymeric materials may include, for example,polytetrafluoroethylene, fluoroacrylate, fluoroeurathane,fluorosilicone, fluorosilane, modified carbonate, chlorosilanes,silicone, and/or combinations thereof. Ceramics may include, forexample, titanium carbide, titanium nitride, chromium nitride, boronnitride, chromium carbide, molybdenum carbide, titanium carbonitride,electroless nickel, zirconium nitride, fluorinated silicon dioxide,titanium dioxide, tantalum oxide, tantalum nitride, diamond-like carbon,fluorinated diamond-like carbon, and/or combinations thereof.Intermetallic compounds may include, for example, nickel aluminide,titanium aluminide, and/or combinations thereof.

As used herein, an intrinsic contact angle is a static contact angleformed between a liquid and a perfectly flat, ideal surface. This angleis typically measured with a goniometer. The following publications,which are hereby incorporated by reference herein in their entireties,describe additional methods for measuring the intrinsic contact angle:C. Allain, D. Aussere, and F. Rondelez, J. Colloid Interface Sci., 107,5 (1985); R. Fondecave, and F. Brochard-Wyart, Macromolecules, 31, 9305(1998); and A. W. Adamson, Physical Chemistry of Surfaces (New York:John Wiley & Sons, 1976).

When a liquid droplet impacts a non-wetting surface, the droplet willspread out on the surface and then begin to recoil. For highlynon-wetting surfaces, the droplet can completely rebound from thesurface. Through the impact dynamics, the shape of the droplet isgenerally axisymmetric so that, at any point in time during recoil, thewetted area is substantially circular. By patterning the surface,however, this symmetry may be disrupted and the impact dynamics may bealtered or controlled. For example, by controlling or definingmacro-scale features on the surface, the contact time of the droplet maybe increased or decreased, instabilities may be created that cause thedroplet to break-up into smaller droplets, and spatial control may begained over how long a particular drop, or part of that drop, is incontact with the surface.

During the time of contact between a droplet and a surface, heat, mass,and momentum diffuse between the droplet and the surface. By controllingthe time that a droplet contacts a particular location on the surface,this diffusion may be optimized both temporally and spatially. Incertain embodiments, surface patterns or features are developed thatinfluence the recoil of droplets in two distinct ways: (1) patterns thatintroduce concavity to the receding boundary, and (2) patterns thatintroduce surface curvature to the film in such a way that capillarypressure delaminates the spread-out droplet from the surface.

The speed at which a spread-out droplet recedes depends not only on thematerial properties of the droplet, but also the properties of thesurface the droplet contacts. On non-wetting surfaces, the droprecoiling speed is reduced by the dissipation or contact anglehysteresis from the surface. Variations in dissipation may be achievedby changing the structure and/or chemistry of the surface patterns thatform the non-wetting surface. For example, the density of patterns suchas posts can influence the recoiling speed of drops. Dissipation in thesystem may be added using a variety of tools, such as flexiblestructures at various length scales. In addition, while a pattern ofposts can break the symmetry of receding films, the drops may remainconvex.

In certain embodiments, surfaces are designed that introduce concavityinto the receding film. Using these designs, the surfaces are tailoredso that the exposure to droplets in certain regions is longer than it isin other regions. In one embodiment, concavity breaks the film intoseparate drops, and the concavity is augmented by natural capillaryinstabilities. For example, the surface may be patterned so the recoilof the drop in one direction is significantly slower than in aperpendicular direction. The resulting recoil forms a cylinder whichquickly becomes concave and breaks up into droplets via aRayleigh-Plateau type instability.

A limitation in the surface pinning approach is that it may slow downthe drop dynamics. The minimum contact time a drop makes with a surfaceis believed to be minimized when that surface approaches a 180 degreecontact angle with no contact angle hysteresis, the equivalent ofimpacting on a thin air layer. As described herein, however, a shortercontact time is possible using patterned surfaces. Specifically, ifduring the recoiling stage, the contact line increases while the surfacearea decreases, there are more fronts on which the droplet can recoil.It is therefore possible for the drop to recede more quickly than if thedrop were receding symmetrically, so that the total contact time for thedrop is reduced. As described below, in certain embodiments, concavityis introduced by speeding up the recoil of portions of the recedingfilm.

FIGS. 1 a and 1 b depict side and top views, respectively, of a waterdroplet 100 bouncing on a superhydrophobic surface 102. The surface 102includes an array of 10 μm square posts of silicon spaced 3 μm apart.The contact time in this case, measured from the leftmost image to therightmost in these figures, is about 19 ms. The scale bar 104 in theleftmost image of FIG. 1 a is 3 mm. FIG. 1 b shows that the dropletspreads and recedes with a largely symmetrical (circular) edge 106.

In certain embodiments, the devices and methods presented herein reducethe contact time between an impinging droplet and a surface by modifyingsurface textures associated with the surface. Surprisingly, thesedevices and methods reduce the contact time to below the theoreticallimit indicated by Equation 1, above. In one embodiment, byappropriately designing the superhydrophobic surface, contact times arefurther decreased to about one half of this theoretical limit.

In certain embodiments, the devices and methods described hereinincorporate macro-scale features (e.g., ridges, sinusoids, protrusions)into a superhydrophobic surface to trigger controlled asymmetry in theliquid film produced by droplet impingement. The macro-scale featuresmay have, for example, a height greater than about 0.00001 mm, greaterthan about 0.0001 mm, greater than about 0.001 mm, greater than about0.01 mm, greater than about 0.1 mm, or greater than about 1 mm.Additionally, the macro-scale features may have, for example, a spacing(e.g., a spacing between ridges, peaks, or valleys) greater than about0.00001 mm, greater than about 0.0001 mm, greater than about 0.001 mm,greater than about 0.01 mm, greater than about 0.1 mm, or greater thanabout 1 mm.

Referring to FIGS. 2 a-2 d, the asymmetry in a liquid film 200, in theform of cracks 204, holes 202, and curvature, introduced by themacro-scale features, leads to droplet recoiling at multiple fronts and,hence, produces a significant reduction in the contact time. This ideais distinctly different from previous approaches which typicallyincluded smaller features (e.g., 100 nm) and, more importantly,attempted to minimize the contact line pinning between the drop andthese features.

In one embodiment, a superhydrophobic surface 300 includes macro-scaleridges 302 that trigger cracks in a liquid film upon impingement of adroplet having radius R. As depicted in FIG. 3, the ridges 302 have aridge height A_(r) and a ridge spacing λ_(r). The ridges 302 may haveany cross-sectional shape, including curved and pointed (as shown inFIG. 3), triangular, hemispherical, and/or rectangular. Typically, eachridge 302 has a ridge length (along the surface 300) that is muchgreater than the ridge height A_(r) and/or ridge spacing λ_(r). Forexample, a ridge 302 may have a ridge height A_(r) of about 0.1 mm and aridge length (e.g., along a ridge longitudinal axis) of about 100 mm ormore. To achieve or maintain superhydrophobicity, the surface 300includes non-wetting features 304 having a length scale L_(n) (e.g., anaverage diameter or cross-dimension). In certain embodiments, thenon-wetting features 304 are chosen so that θ_(d) is greater than 90degrees and CAH is less than about 30 degrees, less than about 20degrees, or less than about 10 degrees. As depicted, the non-wettingfeatures may include smaller features 306, if necessary, to facilitatenon-wetting.

Referring again to FIGS. 1 b and 1 c, when a liquid droplet impinges asolid surface, the droplet spreads into a thin lamella or film having athickness h. In certain embodiments, a ratio of the ridge height A_(r)to the thickness h (i.e., A_(r)/h) is greater than about 0.01. Forexample, A_(r)/h may be from about 0.01 to about 100, from about 0.1 toabout 10, or from about 0.1 to about 5. In certain embodiments, a ratioof the ridge spacing λ_(r) to the ridge height A_(r) is greater than orequal to about 1.

FIGS. 4 and 5 are schematic diagrams showing a droplet 400 recoiling ona flat surface 402 and a droplet 500 recoiling on a ridge 502,respectively. As depicted, on the flat surface 402 of FIG. 4, dropletrecoil is typically symmetric, with the droplet 400 remainingsubstantially circular over time. By comparison, on the ridge 502 ofFIG. 5, droplet recoil is asymmetric, with thinner portions 504 (havingthickness h₁) at the ridge 502 recoiling faster than thicker portions506 (having thickness h₂) adjacent to the ridge 502. The thinnerportions 504 may be referred to as cracks. As depicted, the ridges 502create cracks or pathways that promote droplet fracture. These pathwayscause the contact line to penetrate into the droplet 500 along the ridge502, thereby increasing the contact line length during droplet recoiland reducing contact time.

FIGS. 6 a-6 d and 7 a-7 c depict experimental examples of surfaces fortriggering cracks in a liquid film upon droplet impingement, inaccordance with certain embodiments of the invention. FIGS. 6 a-6 d showphotographs of droplet impingement on a ridge 600 fabricated on asilicon surface 602 using laser-rastering. FIGS. 7 a-7 c show dropletimpingement on a ridge 700, of similar dimensions, milled on an aluminumsurface 702, followed by anodization to create nano-scale pores. Bothsurfaces 602, 702 were made superhydrophobic by depositingtrichloro(1H,1H,2H,2H-perfluorooctyl)silane. The diameter of the dropletbefore impingement was 2.6 mm (i.e., R=1.3 mm) and the impact velocitywas 1.8 m/s.

FIGS. 6 a-6 c show the details of the silicon surface 602 with the helpof SEM images of the ridge 600, which had a ridge height A_(r) of about150 μm and width W of about 200 μm. These figures also show thenon-wetting features achieved to maintain superhydrophobicity. Thedynamics of droplet impingement are shown in FIG. 6 d, which revealsthat a droplet 604 deforms asymmetrically and develops a crack 606 alongthe ridge 600. The crack 606 creates additional recoiling fronts whichpropagate rapidly along the ridge 600 until the film is split intomultiple drops 608. The contact time in this case was only 7 ms—almostone-third of the contact time for the example shown in FIG. 1, and about50% less than the theoretical prediction from Equation 1 (i.e., 13.5 ms)with φ=0.

As mentioned above, the ridges may have any cross-sectional shape,including the approximately rectangular cross-section depicted in FIG. 6a. Additionally, a ratio of the ridge height A_(r) to the width W (i.e.,A_(r)/W) may be, for example, from about 0.1 to about 10.

FIGS. 7 a-7 c show similar contact time reduction achieved on theanodized aluminum oxide (AAO) surface 702. The contact time in this casewas about 6.3 ms, which is over 50% smaller than the theoreticalprediction of Equation 1 (i.e., 13.5 ms). The details of the surface 702are shown in FIGS. 7 a and 7 b with the help of SEM images revealing theridge texture and the nanoporous structure. The scale bars 704, 706 inFIGS. 7 a and 7 b are 100 μm and 1 μm, respectively. Referring to FIG. 7c, the dynamics of droplet impingement show behavior similar to thatseen on the laser-rastered silicon surface. For example, a droplet 708deforms asymmetrically with a crack 710 developing along the ridge 700,thereby causing the liquid film to recoil rapidly along the ridge 700and split into multiple drops 712.

In certain embodiments, the reduction of contact time, as shown in theexamples in FIGS. 6 a-6 d through 7 a-7 c, is more a result of surfacedesign or structure, rather than the surface material or other surfaceproperty. For example, although the surfaces in these examples wereproduced by completely different methods (i.e., laser-rastering in FIG.6 a-6 d, and milling and anodizing in FIGS. 7 a-7 c), the similarmacro-scale features (e.g., ridge size and shape) of the two surfacesresulted in similar drop impingement dynamics.

In another embodiment, a superhydrophobic surface 800 includesmacro-scale protrusions 802 that nucleate holes in a liquid film uponimpingement of a droplet having radius R. The protrusions 802 may haveany shape, including spherical, hemispherical, dome-shaped, pyramidal,cube-shaped, and combinations thereof. For example, in the embodimentdepicted in FIG. 8, the protrusions 802 are substantially dome-shapedwith a protrusion height A_(p) and are spaced in grid with a protrusionspacing λ_(p). To achieve or maintain superhydrophobicity, the surface800 includes non-wetting features having a length scale L_(n). Asmentioned above, the non-wetting features are chosen so that θ_(d) isgreater than 90 degrees and CAH is less than about 30 degrees, less thanabout 20 degrees, or less than about 10 degrees.

In certain embodiments, a ratio of the protrusion height A_(p) to thelamella or film thickness h (i.e., A_(p)/h) is greater than or equal toabout 0.01. For example, A_(p)/h may be from about 0.01 to about 100, orfrom about 0.1 to about 10, or from about 0.1 to about 3. In certainembodiments, a ratio of the protrusion spacing 2 to the protrusionheight A_(p) (i.e., λ_(p)/A_(p)) is greater than or equal to about 2.

FIGS. 9 a-9 c depict an example surface 900 that includes macro-scaleprotrusions 902 for nucleating a droplet upon impingement. The surface900 in this example is made of anodized titanium oxide (ATO). Details ofthe surface 900 are shown in the SEM images. The scale bars 904, 906 inFIGS. 9 a and 9 b are 100 μm and 4 μm, respectively. As depicted, thesurface includes macro-scale protrusions 902, of about 20-100 μm, whichfurther contain non-wetting features to maintain superhydrophobicity.Referring to the high-speed photography images in FIG. 9 c, after adroplet 908 impinges the ATO surface (at t=0), the droplet 908 spreadsinto a thin film (at t=2 ms) that destablizes internally and nucleatesinto several holes 910 (at t=4 ms). The holes 910 grow until theirboundaries meet or collide, thereby causing fragmentation of the entirefilm. Each hole 910 creates additional fronts where the film may recoil,thus resulting in a significant reduction in contact time. The contacttime in this example was about 8.2 ms, which is again much smaller thanthe theoretical prediction (i.e., 13.5 ms) from Equation 1 with φ=0.

In the depicted embodiments, the protrusions increase the contact lineof the droplet by introducing holes in the droplet. The holes increaseor open during recoil, thereby reducing the contact time.

In another embodiment, a superhydrophobic surface 1000 includesmacro-scale curved profiles 1002 that introduce curvature in a liquidfilm upon impingement of a droplet having radius R. The curved profiles1002 may have any shape, including sinusoidal and/or parabolic (e.g.,piece-wise). Compared to the ridges 302 and protrusions 802, describedabove, the curved profiles 1002 are generally smoother, with less abruptvariations in surface height. For example, in the embodiment depicted inFIG. 10, the curved profiles 1002 define a sinusoidal pattern of peaksand valleys on the surface. The sinusoidal pattern has a wave amplitudeA_(c) and a wave spacing λ_(c) (i.e., the distance from a peak to avalley). The wave spacing λ_(c) may also be referred to as half theperiod of the sinusoidal pattern.

In certain embodiments, the surface 1000 includes curvature along morethan one direction. For example, a height of surface 1000 may varysinusoidally along one direction and sinusoidally along another,orthogonal direction.

To achieve or maintain superhydrophobicity, the surface 1000 includesnon-wetting features having a length scale L_(n). As mentioned above,the non-wetting features are chosen so that θ_(d) is greater than 90degrees and CAH is less than about 30 degrees, less than about 20degrees, or less than about 10 degrees.

In certain embodiments, a ratio of the wave amplitude A_(c) to thethickness h (i.e., A_(c)/h) is greater than or equal to about 0.01. Forexample, A_(c)/h may be from about 0.01 to about 100, or from about 0.1to about 100, or from about 0.1 to about 50, or from about 0.1 to about9. In certain embodiments, a ratio of the wave spacing λ_(c) to the waveamplitude A_(c) (i.e., ζ_(c)/A_(c)) is greater than or equal to about 2.For example, λ_(c)/A_(c) may be from about 2 to about 500, or from about2 to about 100.

FIG. 11 a depicts an example of a sinusoidal curved surface 1100fabricated on silicon using laser rastering. The details of the surface1100 are shown with the help of SEM images. The wave amplitude A_(c) ofthe sinusoidal pattern was about 350 μm while its period (i.e., twicethe wave spacing λ_(c)) was 2 mm. The surface 1100 was madesuperhydrophobic by depositingtrichloro(1H,1H,2H,2Hperfluorooctyl)silane. Referring to FIG. 11 b, thedynamics of droplet impingement on the surface 1100 reveal that adroplet 1102 adopts the curved profile of the surface 1100 whilespreading and becomes a thin film of varying thickness. The filmthickness is smallest at a crest or peak 1104 of the sinusoidal surface1100 where the film recedes fastest, thereby causing the film to splitacross the crest 1104 and break into multiple drops 1106. The contacttime in this example was only about 6 ms, which is again well over 50%smaller than the theoretical prediction of Equation 1 (i.e., 13.5 ms).

As described above with respect to FIGS. 10, 11 a, and 11 b, in certainembodiments, the contact time of the drop is reduced by controlling thelocal curvature of the surface. If the surface is curved so that part ofthe film covers a concave region, one of two scenarios may occur—both ofwhich decrease the total contact time of the film on the surface. In onescenario, the film spreads over the concavity so that the thickness isnearly uniform. If the film is making contact with the curved surface,then the film is also curved, in which case the film curvature, alongwith surface tension, causes a pressure gradient that lifts the film offof the surface as quickly as the edges recoil. In the other scenario,the film spreads over the concavity in a way that the film surface isflat (i.e., not curved). In this case the film thickness is not uniformand, along contours where the film is thinner, the drop recoils morequickly than along areas where the film is thicker. As discussed above,by forming a hybrid surface of linked concave cusps, the contact timemay be reduced below the theoretical limit defined by Equation 1.

When a liquid droplet 1200 of diameter D_(o) impinges a solid surface1202 with velocity V_(o), the droplet 1200 spreads into a thin lamella(film) 1204 of thickness h, eventually reaching a maximum diameterD_(max), as shown in FIGS. 12 a, 12 b, and 12 c. h can be estimated byapplying mass conservation at the spherical droplet state, shown in FIG.12 a, and the lamella state, shown in FIG. 12 c, with the assumptionsthat there is negligible mass loss (e.g., due to splashing orevaporation) during spreading and the lamella 1204 is substantiallyuniform in thickness in time and space, on average. With theseassumptions, the mass of the droplet 1200 when equated at the sphericaldroplet state and the lamella state yields:

$\begin{matrix}{{{\rho \frac{\pi}{6}D_{o}^{3}} = {\rho \frac{\pi}{4}D_{\max}^{2}h}},} & (3)\end{matrix}$

where ρ is the density of droplet liquid. Solving Equation 3 for hgives:

$\begin{matrix}{{h = \frac{2\; D_{o}}{3\; \xi_{\max}^{2}}},} & (4)\end{matrix}$

where ξ_(max)=D_(max)/D_(o) is the maximum spread factor of theimpinging droplet. To calculate ξ_(max), an energy balance model may beused. According to this model, ξ_(max) is given as:

$\begin{matrix}{{\xi_{\max} = \sqrt{\frac{{We} + 12}{{3( {1 - {\cos \; \theta_{a}}} )} + {4( {{We}/\sqrt{Re}} )}}}},} & (5)\end{matrix}$

where θ_(a) is the advancing contact angle formed by a droplet of liquidon the solid surface 1202, We=ρV_(o) ²D_(o)/γ is the droplet Webernumber, and Re=ρV_(o)/D_(o)/μ is the droplet Reynolds number beforeimpingement. Here γ and μ are the surface tension and dynamic viscosityof the droplet liquid, respectively. Equation 5 can be simplifiedfurther by approximating the value of expression 3(1−cos θ_(a)) to 6 asθ_(a), at maximum, can be 180°. With this simplification, Equation 5becomes:

$\begin{matrix}{{\xi_{\max} = \sqrt{\frac{{We} + 12}{6 + {4( {{We}/\sqrt{Re}} )}}}},} & (6)\end{matrix}$

Thus, once ξ_(max) is calculated from Equation 6, h can be estimatedusing Equation 4.

The devices and methods described herein have a wide range ofapplications, including rainproof products, wind turbines, steam turbineblades, aircraft wings, and gas turbine blades. Table 1 presents typicaldroplet radius values for several of these applications. As indicated,for rainproof products and wind turbine applications, droplet radiusvalues may be from about 0.1 mm to about 5 mm. Similarly, for steamturbine blades, aircraft icing, and gas turbine blade applications,droplet radius values may be from about 0.01 mm to about 5 mm. In oneembodiment, for rainproof products and wind turbine applications,lamella thickness values are from about 0.01 mm to about 1 mm, andξ_(max) values are from about 5 to about 100. In another embodiment, forsteam turbine blades, aircraft icing, and gas turbine bladeapplications, lamella thickness values are from about 0.001 mm to about1 mm, and ξ_(max) values are from about 10 to about 500.

In certain embodiments, Table 1 is used to identify appropriatedimensions for the features described above (i.e., ridges, protrusions,and curved profiles) for reducing the contact time between an impingingdroplet and a surface. For example, referring to Table 1, if theintended application is rainproof products and the feature type isridges, then appropriate feature dimensions (in mm) are 0.0001<A_(r) andλ_(r)≧0.0001. Likewise, if the intended application is gas turbineblades and the feature type is protrusions, then appropriate featuredimensions (in mm) are 0.00001<A_(p) and λ_(p)≧0.00002.

As indicated in Table 1, A_(r), A_(p), or A_(c) may be greater than0.00001 mm, and λ_(r), λ_(p), or λ_(c) may be greater than or equal toabout 0.00001 mm. In certain embodiments, A_(r), A_(p), or A_(c) isgreater than about 0.0001 mm, greater than about 0.001 mm, greater thanabout 0.01 mm, greater than about 0.1 mm, or greater than about 1 mm. Incertain embodiments, A_(r), A_(p), or A_(c) is from about 0.00001 mm toabout 0.001 mm, from about 0.0001 mm to about 0.01 mm, from about 0.001mm to about 0.1 mm, or from about 0.01 mm to about 1 mm. In certainembodiments, λ_(r), λ_(p), or λ_(c) is greater than about 0.0001 mm,greater than about 0.001 mm, greater than about 0.01 mm, greater thanabout 0.1 mm, or greater than about 1 mm. In certain embodiments, λ_(r),λ_(p), or λ_(c) is from about 0.00001 mm to about 0.001 mm, from about0.0001 mm to about 0.01 mm, from about 0.001 mm to about 0.1 mm, or fromabout 0.01 mm to about 1 mm.

TABLE 1 Ranges for droplet radius and macro-scale feature dimensions.Lamella Droplet Impact Thick- Feature Appli- Radius, Velocity, ness,Feature Dimensions* cation R (mm) V (m/s) h (mm) Type (mm) Rainproof 0.1-5 0.5-20   0.01-1 Type (i): 0.0001 < A_(r,) products ridges λ_(r) ≧0.0001 & wind Type (ii): 0.0001 < A_(p), turbine protrusions λ_(p) ≧0.0002 Type (iii): 0.0001 < A_(c), curvature 0.0002 ≦ λ_(c) Steam 0.01-50.5-200 0.001-1 Type (i): 0.00001 < A_(r), turbine ridges λ_(r) >0.00001 blades, Type (ii): 0.00001 < A_(p), Aircraft protrusions λ_(p) ≧0.00002 icing, Gas Type (iii): 0.00001 < A_(c), turbine curvature0.00002 ≦ λ_(c) blades

In alternative embodiments, the devices and methods described hereinapply to droplets of oil-based liquids impinging on an oleophobicsurface or a superoleophobic surface. In this case, the macro-scalefeatures, such as ridges, protrusions, and sinusoidal patterns, mayproduce oil droplet impingement dynamics that are similar to those shownand described for water droplets impinging a hydrophobic orsuperhydrophobic surface.

In certain embodiments, when a water droplet impinges a surface that ishot enough to vaporize the liquid quickly and generate sufficientpressure, the droplet can spread and rebound without ever touching thesurface, mimicking a situation seen in superhydrophobic surfaces. Thisso-called Leidenfrost phenomenon is an example of a non-wettingsituation without the surface being superhydrophobic. In one embodiment,the macro-scale features applied to this type of surface are effectivein reducing the contact time of an impinging droplet. Specifically, thedroplet dynamics are similar to those described above for thesuperhydrophobic surfaces, and the contact time reduction is of similarmagnitude (˜50% of the theoretical limit). In one embodiment, to achievethe desired non-wetting behavior, the surface is heated to a temperaturegreater than the Leidenfrost temperature.

Blades of steam and gas turbines are sometimes fouled by metallicfragments that are produced due to erosion/corrosion of intermediaryequipment in the power cycle. These fragments are carried along with theworking fluid (steam or combustion gases, as the case may be) and meltwhen they reach regions of high temperatures. The melted liquid impingesupon turbine blades and gets stuck thereby deteriorating aerodynamicalperformance and hence turbine power output. Our surface designs cansolve this problem by rapidly repelling the impinging molten liquidbefore it can freeze on blade surfaces.

Experimental Examples

As described herein, a series of experiments were conducted to measureand visualize the impingement of droplets on surfaces having macro-scalefeatures. A high speed camera system (Model SA 1.1, PHOTRON USA, SanDiego, Calif.) was utilized to capture a sequence of images of thedroplet impingement. Droplets of controlled volume (10 μL) weredispensed using a syringe pump (HARVARD APPARATUS, Holliston, Mass.)using a 26 gauge stainless steel needle. Droplet impact velocity wascontrolled by setting the needle at a certain height (150 mm) above thesurface. Contact times were determined from the images by identifyingthe time difference between the point of initial droplet contact withthe surface and the subsequent rebound of liquid from the surface.

Images of macro-scale ridges and droplets impinging on the ridges areprovided in FIGS. 6 a-6 d and 7 a-7 c, in accordance with certainembodiments of the invention. FIGS. 6 a-6 d show photographs of dropletimpingement on a ridge 600 fabricated on a silicon surface 602 usinglaser-rastering. FIGS. 7 a-7 c show droplet impingement on a ridge 700,of similar dimensions, milled on an aluminum surface 702, followed byanodization to create nano-scale pores. Both surfaces 602, 702 were madesuperhydrophobic by depositingtrichloro(1H,1H,2H,2H-perfluorooctyl)silane. The diameter of the dropletbefore impingement was 2.6 mm (i.e., R=1.3 mm) and the impact velocitywas 1.8 m/s. As discussed in detail above, the contact times achievedwith the macro-scale ridges were about 50% less than the theoreticalprediction from Equation 1 (i.e., 13.5 ms) with φ=0.

Images of macro-scale protrusions and droplets impinging on theprotrusions are provided in FIGS. 9 a-9 c, in accordance with certainembodiments of the invention. The surface 900 in this example is made ofanodized titanium oxide (ATO). Details of the surface 900 are shown inthe SEM images. The scale bars 904, 906 in FIGS. 9 a and 9 b are 100 μmand 4 μm, respectively. As depicted, the surface includes macro-scaleprotrusions 902, of about 20-100 μm, which further contain non-wettingfeatures to maintain superhydrophobicity. As discussed in detail above,the contact times achieved with the macro-scale protrusions was abouthalf of the theoretical prediction (i.e., 13.5 ms) from Equation 1 withφ=0.

Images of macro-scale curvature and droplets impinging on the curvatureare provided in FIGS. 11 a and 11 b, in accordance with certainembodiments of the present invention. As discussed above, the sinusoidalcurved surface 1100 was fabricated on silicon using laser rastering. Thedetails of the surface 1100 are shown with the help of SEM images. Thewave amplitude A_(c) of the sinusoidal pattern was about 350 μm whileits period (i.e., twice the wave spacing λ_(c)) was 2 mm. The surface1100 was made superhydrophobic by depositingtrichloro(1H,1H,2H,2Hperfluorooctyl)silane. The contact time in thisexample was only about 6 ms, which is again well over 50% smaller thanthe theoretical prediction of Equation 1 (i.e., 13.5 ms).

EQUIVALENTS

While the invention has been particularly shown and described withreference to specific preferred embodiments, it should be understood bythose skilled in the art that various changes in form and detail may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is: 1-42. (canceled)
 43. An atomizer comprising anon-wetting surface having a dynamic contact angle of at least about90°, said surface patterned with macro-scale features configured toinduce controlled asymmetry in a liquid film produced by impingement ofa droplet onto the surface, thereby promoting breakup of the droplet onthe surface.
 44. The atomizer of claim 43, wherein the non-wettingsurface is supermetallophobic.
 45. The atomizer of claim 43, wherein thedroplet comprises a molten metal.